Displacement measuring interferometers monitor changes in the position of a measurement object relative to a reference object based on an optical interference signal. The interferometer generates the optical interference signal by overlapping and interfering a measurement beam reflected from the measurement object with a reference beam reflected from the reference object.
In many applications, the measurement and reference beams have orthogonal polarizations and different frequencies. The different frequencies can be produced, for example, by laser Zeeman splitting, by acousto-optical modulation, or internal to the laser using birefringent elements or the like. The orthogonal polarizations allow a polarizing beam splitter to direct the measurement and reference beams to the measurement and reference objects, respectively, and combine the reflected measurement and reference beams to form overlapping exit measurement and reference beams. The overlapping exit beams form an output beam that subsequently passes through a polarizer.
The polarizer mixes polarizations of the exit measurement and reference beams to form a mixed beam. Components of the exit measurement and reference beams in the mixed beam interfere with one another so that the intensity of the mixed beam varies with the relative phase of the exit measurement and reference beams. A detector measures the time-dependent intensity of the mixed beam and generates an electrical interference signal proportional to that intensity. Because the measurement and reference beams have different frequencies, the electrical interference signal includes a “heterodyne” signal having a beat frequency equal to the difference between the frequencies of the exit measurement and reference beams. If the lengths of the measurement and reference paths are changing relative to one another, e.g., by translating a stage that includes the measurement object, the measured beat frequency includes a Doppler shift equal to 2νnp/λ, where ν is the relative speed of the measurement and reference objects, λ is the wavelength of the measurement and reference beams, n is the refractive index of the medium through which the light beams travel, e.g., air or vacuum, and p is the number of passes to the reference and measurement objects. Changes in the relative position of the measurement object correspond to changes in the phase of the measured interference signal, with a 2π phase change substantially equal to a distance change L of λ/(np), where L is a round-trip distance change, e.g., the change in distance to and from a stage that includes the measurement object.
Unfortunately, this equality is not always exact. In addition, the amplitude of the measured interference signal may be variable. A variable amplitude may subsequently reduce the accuracy of measured phase changes. Many interferometers include non-linearities such as what are known as “cyclic errors.” The cyclic errors can be expressed as contributions to the phase and/or the intensity of the measured interference signal and have a sinusoidal dependence on the change in optical path length pnL. In particular, the first harmonic cyclic error in phase has a sinusoidal dependence on (2πpnL)/λ and the second harmonic cyclic error in phase has a sinusoidal dependence on 2(2πpnL)/λ. Higher harmonic cyclic errors and sub-harmonic errors can also be present.
Cyclic errors can be produced by “beam mixing,” in which a portion of an input beam that nominally forms the reference beam propagates along the measurement path and/or a portion of an input beam that nominally forms the measurement beam propagates along the reference path. Such beam mixing can be caused by misalignment of interferometer with respect to polarization states of input beam, birefringence in the optical components of the interferometer, and other imperfections in the interferometer components, e.g., imperfections in a polarizing beam-splitter used to direct orthogonally polarized input beams along respective reference and measurement paths. Because of beam mixing and the resulting cyclic errors, there is not a strictly linear relation between changes in the phase of the measured interference signal and the relative optical path length pnL between the reference and measurement paths. If not compensated, cyclic errors caused by beam mixing can limit the accuracy of distance changes measured by an interferometer. Cyclic errors can also be produced by imperfections in transmissive surfaces that produce undesired multiple reflections within the interferometer and imperfections in components such as retroreflectors and/or phase retardation plates that produce undesired ellipticities and undesired rotations of planes of polarization in beams in the interferometer. For a general reference on the theoretical causes of cyclic errors, see, for example, C. W. Wu and R. D. Deslattes, “Analytical modelling of the periodic nonlinearity in heterodyne interferometry,” Applied Optics, 37, 6696–6700, 1998.
One example of a sub-harmonic cyclic error is generated when the measurement object mirror is parallel to the conjugate image of the reference mirror and, for a typical wavelength, leads to an error in the distance measured with an amplitude between 2.5 nm and approximately 5 nm depending on the amount of relative beam shear between the reference and measurement beams at a detector. The amplitude of one of the harmonic cyclic errors so generated is 0.25 nm to approximately 0.5 nm in a double pass plane mirror interferometer depending on the amount of relative beam shear. The sub-harmonic cyclic error can be eliminated by orienting the interferometer such the condition for generation of the sub-harmonic error is not met such as described in commonly owned U.S. Provisional Application No. 60/314,490 [Z-337] filed Aug. 23, 2001 and entitled “Tilted Interferometer” by Henry A. Hill. However, the harmonic cyclic error cannot be eliminated by such a procedure and can present a problem in lithography applications requiring high precision of the order of 0.1 nm such as EUV lithography.
The interferometers described above are often crucial components of scanner systems and stepper systems used in lithography to produce integrated circuits on semiconductor wafers. Such lithography systems typically include a translatable stage to support and fix the wafer, focusing optics used to direct a radiation beam onto the wafer, a scanner or stepper system for translating the stage relative to the exposure beam, and one or more interferometers. Each interferometer directs a measurement beam to, and receives a reflected measurement beam from, a plane mirror attached to the stage. Each interferometer interferes its reflected measurement beams with a corresponding reference beam, and collectively the interferometers accurately measure changes in the position of the stage relative to the radiation beam. The interferometers enable the lithography system to precisely control which regions of the wafer are exposed to the radiation beam.
In many lithography systems and other applications, the measurement object includes one or more plane mirrors to reflect the measurement beam from each interferometer. Small changes in the angular orientation of the measurement object, e.g., pitch and yaw of a stage, can alter the direction of each measurement beam reflected from the plane mirrors. If left uncompensated, the altered measurement beams reduce the overlap of the exit measurement and reference beams in each corresponding interferometer. Furthermore, these exit measurement and reference beams will not be propagating parallel to one another nor will their wave fronts be aligned when forming the mixed beam. As a result, the interference between the exit measurement and reference beams will vary across the transverse profile of the mixed beam, thereby corrupting the interference information encoded in the optical intensity measured by the detector.
To address this problem, many conventional interferometers include a retroreflector that redirects the measurement beam back to the plane mirror so that the measurement beam “double passes” the path between the interferometer and the measurement object. The presence of the retroreflector insures that the direction of the exit measurement is insensitive to changes in the angular orientation of the measurement object. When implemented in a plane mirror interferometer, the configuration results in what is commonly referred to as a high-stability plane mirror interferometer (HSPMI). Retroreflectors are also useful in systems employing other interferometer configurations.
One type of retroreflector used in such systems is a cube corner retroreflector. Such a retroreflector contains three mutually perpendicular surfaces which together reflect light along an output path that is anti-parallel to the input path. A light beam may first hit any one of the three surfaces over a large range of angles and is subsequently reflected towards the second and third surfaces, leaving in the reversed direction with a lateral displacement. The three surfaces can be part of a solid cube corner where the light hits the surface from inside a solid material with air on the other side, or they can be part of a hollow cube corner hitting the surface of a material from air. The hollow cube corner may have loss associated with the reflecting surfaces. A solid cube corner can use total internal reflection at the three surfaces to reduce reflection losses, but may introduce loss or back-scattering at its input face due to an imperfect anti-reflection coating. In general, the polarization state of a beam is changed upon exiting the cube corner retroreflector. The two main effects are rotation of the polarization and ellipticity. Both effects can cause beam mixing, resulting in cyclic errors. The polarization can can also be affected by reflection losses. As described in an article by N. Bobroff entitled “Recent Advances In Displacement Measuring Interferometry, “Measurement Science & Technology,” 4(9), pp 907–926 (1993), a solid cube corner retroreflector coated with silver, common in commercial interferometers, rotates the polarization by 0.1 rad. Such a rotation is a source a sub-harmonic and harmonic cyclic errors in a double pass plane mirror heterodyne interferometer.
In practice, the interferometry systems are used to measure the position of the wafer stage along multiple measurement axes. For example, defining a Cartesian coordinate system in which the wafer stage lies in the x-y plane, measurements are typically made of the x and y positions of the stage as well as the angular orientation of the stage with respect to the z axis, as the wafer stage is translated along the x-y plane. Furthermore, it may be desirable to also monitor tilts of the wafer stage out of the x-y plane. For example, accurate characterization of such tilts may be necessary to calculate Abbe offset errors in the x and y positions. Thus, depending on the desired application, there may be up to five degrees of freedom to be measured. Moreover, in some applications, it is desirable to also monitor the position of the stage with respect to the z-axis, resulting in a sixth degree of freedom.
To measure each degree of freedom, an interferometer is used to monitor distance changes along a corresponding metrology axis. For example, in systems that measure the x and y positions of the stage as well as the angular orientation of the stage with respect to the x, y, and z axes, at least three spatially separated measurement beams reflect from one side of the wafer stage and at least two spatially separated measurement beams reflect from another side of the wafer stage. See, e.g., U.S. Pat. No. 5,801,832 entitled “Method of and Device for Repetitively Imaging a Mask Pattern on a Substrate Using Five Measuring Axes,” the contents of which are incorporated herein by reference. Each measurement beam is recombined with a reference beam to monitor optical path length changes along the corresponding metrology axes. Because the different measurement beams contact the wafer stage at different locations, the angular orientation of the wafer stage can then be derived from appropriate combinations of the optical path length measurements. Accordingly, for each degree of freedom to be monitored, the system includes at least one measurement beam that contacts the wafer stage. Furthermore, as described above, each measurement beam may double-pass the wafer stage to prevent changes in the angular orientation of the wafer stage from corrupting the interferometric signal. The measurement beams may generated from physically separate interferometers or from multi-axes interferometers that generate multiple measurement beams.